1. Field of the Invention
The present invention relates to a MOS type semiconductor device, and more particularly, to a potential stabilizing circuit connected between power voltage supply lines of different potentials each other.
2. Description of Related Art
To suppress a potential fluctuation on power voltage supply lines (Vcc line and ground line, for example) caused by an operation of a signal generation circuit, etc., a potential stabilizing circuit including a capacitor called as bypass capacitor or decoupling capacitor is connected between these power voltage supply lines, because the potential fluctuation adversely affects operation speeds, etc. of the device, and in the worst case, it causes erroneous operations.
The bypass capacitor is favorably of a MOS type capacitor when the semiconductor device includes a plurality of insulated gate field effect transistors (MOS type transistors), because the MOS type capacitor can be manufactured with the transistors in the same process steps thereby obtaining a good productivity. Consequently, the dielectric film of the MOS capacitor has the same thickness and material as the gate insulating film of the transistor. Therefore, when the transistor necessitates a thinner gate insulating film such as 200 .ANG. to 400 .ANG. to meet a demand of high integration density of device, the dielectric film of the bypass capacitor becomes inevitably the same thin thickness.
In a prior art, the potential stabilizing circuit is constituted such that an upper electrode of a MOS type capacitor is connected to a Vcc (+5 volts) power supply line; a lower electrode of the MOS type capacitor, which lower electrode is an impurity region formed in the substrate, is connected to a ground (0 volts) line; and the dielectric film of the MOS type capacitor between the upper and lower electrodes is made of silicon oxide and has the same thin thickness as the silicon oxide gate insulating film of the transistor.
The semiconductor devices forming the conventional potential stabilizing circuit mentioned above, however, represent low reliability through reliability estimation tests or acceleration tests (operating the device with a higher power supply voltage under a high temperature) causing by a breakdown of the thin dielectric film of the capacitor; although the thin gate insulating films of the transistors maintain good state. The reason is suggested that a high electric field is continuously applied to the dielectric film of the bypass capacitor during the power supply voltages are supplied to the device, that is, to the bonding pads thereof, while it is applied to the gate insulating film of each transistor only when the transistor is in an operating state. To avoid this disadvantage, on the other hand, if the dielectric film is formed thicker than the gate insulating film, complexity of manufacturing of the device is inevitable.